The ever increasing availability and popularity of wireless communication can be linked to technological gains that have provided more efficient, reliable and cost-effective mobile devices, such as wireless digital telephones and personal communication systems ("PCSs"), as examples. Due to their mobility and low power requirements, conventional mobile devices impose significant design constraints upon the wireless communication networks and, more particularly, the switching offices that support them.
Each switching office is associated with multiple transceiver sites, or "cells," that enable communication between the mobile devices and the switching office. Typically, there is a high density, or closeness, of cells per geographic area, often in a honeycomb pattern of overlapping cells of communication. Cell density causes each mobile device to always be "close" to at least one cell. Thus, any wireless signal may be concurrently heard by several cells and, possibly, several switching offices. Each cell generally covers a range of several miles in each direction, which may of course be limited by natural or man made objects--mountains, buildings, etc.
In the past, wireless communication was largely analog-based, but in recent years, the wireless carriers have moved toward digital-based communications. This transition stems from compatibility and frequency utilization perspectives--if users can share a frequency or a range of frequencies, then more users can be accommodated on less bandwidth.
An increasingly popular wireless digital communication methodology is Code Division Multiple Access ("CDMA"). CDMA is a version of "older" spread spectrum technologies. Spread spectrum technology, introduced in the 1920s, has evolved over a number of decades from uses in secured military applications to conventional civilian wireless communication applications.
More particularly, spread spectrum technology provides a means for organizing radio frequency energy over a somewhat wide range of frequencies and moving among the frequency range on a time divided basis. As an example, a transmitter transmits at a first frequency at a first time and at a second frequency at a second time; a receiver receiving these transmissions is synchronized to switch frequencies during reception in response to the change from the first to the second frequency.
Whenever multiple signals are communicated through a communication network, the potential for losing data or degradation of the communication signal may increase exponentially. Maintaining a synchronized signal among a transmitter and a receiver is therefore paramount. If the synchronization, or timing, of transmission or arrival of a signal is off, then the information content of the signal may be distorted or lost--this phenomenon is commonly referred to as "slippage."
Searching for and tracking of a communication signal are therefore two of the most important synchronization processes performed by the receiver. The searching process operates to find or locate possible signal paths in order to demodulate the strongest received communication signal (as well as to provide candidates for soft handoff).
The tracking process, in contrast, operates to track a received communication signal. This is often accomplished using a "tracking loop." Conventional tracking loops work to fine-tune the signal path, most often to a static pseudorandom number ("PN") chip. Since a received signal may be transmitted at any one of a number of data rates, the lowest possible data rate is most often assumed--only a small portion of all the data received is used for the tracking process, resulting in an inefficient tracking loop.
Therefore, what is needed in the art is an adaptive means for tracking a received signal wherein a convergence rate of the tracking means is made to depend upon the data rate of the received signal. What is further needed in the art is a tracking means that follows changes in signal delay due to movement and reference timing adjustments of a mobile device.